Conventionally, ultrasonography is known as a method of nondestructively inspecting a structural member of an aircraft or the like. Furthermore, a technique using an FBG (fiber Bragg grating) sensor as a sensor for ultrasonic testing is also known (for example, refer to Japanese Patent Application Publication JP 2005-257289 A and Japanese Patent Application Publication JP 2006-132952 A). Using an FBG sensor as a sensor for receiving an ultrasonic wave can improve detection accuracy of ultrasonic waves compared to a case where an ultrasonic transducer is used as the sensor.
However, an FBG sensor has reception directivity. Specifically, an FBG sensor can only receive an ultrasonic wave, which propagates toward the approximately longitudinal direction of the fiber. Thereby, when an FBG sensor is used as a sensor for receiving an ultrasonic wave, it is difficult to highly accurately receive not only an ultrasonic wave propagating from the perpendicular direction to the longitudinal direction of the FBG sensor, but also an ultrasonic wave propagating from a direction inclined by more than a certain angle to the longitudinal direction of the FBG sensor.
Therefore, when it is difficult to dispose many FBG sensors or to secure a space for disposing an FBG sensor, there is a problem that an ultrasonic wave cannot be detected using an FBG sensor. Conversely, even when an FBG sensor is disposed, an ultrasonic wave can be detected only in a limited range.
In particular, when a target of an ultrasonic test is an aircraft structural object having reinforcement members, such as spars, ribs, and stringers, attached on a panel (which is also called a skin), it may be difficult to secure a space for disposing an FBG sensor due to interference with the many reinforcement members and cables.
Thus, an object of the present invention is to allow detecting ultrasonic waves with higher accuracy in ultrasonography, without being restricted by the reception directivity of a sensor for receiving an ultrasonic wave and/or a space for disposing the sensor for receiving an ultrasonic wave.